In motor vehicles, petrol travels at high speeds, under the effect of the injection pump, in conduits connecting the engine to the storage tank. In some cases the friction between petrol and the internal wall of the pipe can give rise to electrostatic charges the accumulation of which can result in an electrical discharge (spark) capable of igniting the petrol with catastrophic consequences (explosion). The surface resistivity of the inner face of the pipe must therefore be limited to a value which is generally lower than 10.sup.6 ohms. It is known to lower the surface resistivity of polymer resins or materials by incorporating therein conductive and/or semiconductive materials such as carbon black, steel fibers, carbon fibers or particles (fibers, platelets, spheres, etc.) which are metallized with gold, silver or nickel.
Among these materials, carbon black is employed more particularly, for reasons of economy and ease of processing. Apart from its special electroconductive properties, carbon black behaves as a filler such as, for example, talc, chalk or kaolin. A person skilled in the art thus knows that, when the filler content increases, the viscosity of the polymer/filler mixture increases. Similarly, when the filler content increases, the flexural modulus of the filled polymer increases. These known and predictable phenomena are explained in the "Handbook of Fillers and Reinforcements for Plastics", edited by H. S. Katz and J. V. Milewski, Van Nostrand Reinhold Company, ISBN 0-442-25372-9; see in particular Chapter 2, Section II for fillers in general, and Chapter 16, Section VI for carbon black in particular.
As for the electrical properties of carbon black, the technical note "Ketjenblack EC--Black 94/01" from the Akzo Nobel Company indicates that the resistivity of the formulation falls very abruptly when a critical content of carbon black, called a percolation threshold, is reached. When the carbon black content increases further, the resistivity decreases rapidly until it reaches a stable level (plateau region). For a given resin, therefore, it is preferred to operate in the resistivity plateau region, where an error in metering will only slightly affect the resistivity of the compound. Moreover, metering of products of very low apparent density, like carbon black, is not easy and an error in the metering of the black is always possible.
At present, polyamide pipes are commonly employed for conveying petrol in motor vehicles. In fact, polyamides represent an ideal material for the manufacture of pipes because their mechanical strength is excellent and their flexibility is sufficient for a pipe to withstand, without breaking, practically throughout the life of a vehicle, the accumulation of flexing motions, especially at low temperature. The applicants have found that, in the case of polyamide 12 of inherent viscosity of approximately 1.45 in m-cresol and plasticized with 12.5 mass % of n-butylbenzenesulphonamide (BBSA), the surface resistivity changes little above 9 mass % (beginning of the resistivity plateau region) of carbon black (KETJENBLACK.RTM. EC 600 JD carbon black from Akzo Nobel, characterized by a pore volume of 400 ml of dibutyl phthalate per 100 g of carbon black and by a specific surface area of 1250 m.sup.2 /g, measured by the nitrogen absorption method (BET method)).
However, these pipes made of antistatic polyamide no longer meet the new requirements of motor vehicle manufacturers with regard to permeability. With the increasingly frequent addition of methanol to petrol or gasoline, the sensitivity of the polyamide pipes manifests itself as swelling of the pipe, entailing a decrease in the mechanical properties and dimensional changes.
To overcome this disadvantage, while retaining the mechanical effects of the polyamides, it has been proposed in EP 558373 to sheathe the inner wall of the polyamide pipe with a layer of fluoropolymer, preferably polyvinylidene fluoride (PVDF), the fluoropolymer layer being preferably as thin as possible in order to preserve the flexibility of polyamide as much as possible, it being known that fluoropolymers, and especially PVDF, are not particularly renowned for their properties in respect of suppleness. However, once filled with carbon black, PVDF has very poor impact strength, especially at low temperature, and this solution is therefore unsatisfactory.
Another alternative, proposed in WO 94/29626, consists of a polyamide-based pipe made up of an outer layer of polyamide, a middle layer of fluoropolymer, preferably of PVDF, and an inner layer of polyamide, these layers being bonded to each other respectively by layers of adhesive binder. However, when the PA-based inner layer is filled with carbon black in proportions corresponding to the resistivity plateau region, the PA-adhesive binder adhesion is virtually nil.
This absence of adhesion is all the more surprising since, when carbon black is replaced with another reinforcing filler such as, for example, calcium carbonate (OMYALITE.RTM. 90T from Omya) in the same mass proportion, this new composition based on the same polyamide 12 has an excellent adhesion in coextrusion or in pressing films onto a coextrusion binder.